Grid-tied converters, commonly used in association with the electric grid, convert renewable energy into alternating current (AC) format that electric power system can accept. For example, grid-tied converters are used to convert energy from an energy source, such as solar panel, wind turbine, or some other source, into an AC format for feeding energy into the grid, based upon a relevant local grid code.
As understood by those of skill in the art, grid codes are technical specifications defining specific parameters that an external source, or facility, must achieve to connect to the electric grid. The specifications ensure compatibility, safety, and security of the supplied electricity.
By way of background, power flowing into the electric grid includes two types: active power (P) and reactive power (Q). Grid codes differ by countries, but one common request is the grid-tied renewable energy converters are generally required a reactive power compensation ability which must be under the control of grid management system and some grid codes even stress the priority to use the converters' reactive power capability than static VAR compensator when needed.
Conventional grid-tied converters usually include an ability to do the reactive power compensation. Conventional grid-tied converters, however, lack the ability to forecast converters' own reactive power capabilities accurately, to help the grid management system understanding how much the converters can do in reactive power compensation.
Most of these conventional systems perform reactive power capability prediction in a simple way. For example, one common technique estimates the maximum capability of Q as a function of apparent power (S) and active power (P), as Q=√{square root over (S2−P2)}. This approach, however, fails to consider that Q capability is not simply a function of apparent power and active power (P), but is closely related to the external environment of the electric grid. The conventional systems ignore that Q capability is a function of the electric grid's impedance/voltage/frequency and the power converter's internal states (active power/DC bus voltage limit/control techniques). More specifically, these conventional systems lack a capability to accurately predict maximum Q capability.